Day 21
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194
Day21.scala
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194
Day21.scala
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package aoc.day21
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import aoc._
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import aoc.direction._
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import scala.collection.mutable
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val board =
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lines
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.map(_.toArray)
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.toArray
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val startPoint =
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board.toSeq.zipWithIndex.findMap { (row, idx) =>
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row.toSeq.zipWithIndex.findMap: (cell, col) =>
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if cell == 'S' then Some((idx, col)) else None
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}.get
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def inside(x: Int, y: Int) =
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x >= 0 && x < board.length && y >= 0 && y < board(0).length
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def walkable(x: Int, y: Int) = inside(x, y) && board(x)(y) != '#'
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def getInf(x: Int, y: Int) =
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def mod(x: Int, m: Int): Int = if x < 0 then mod(x % m + m, m) else x % m
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board(mod(x, board.size))(mod(y, board(0).size))
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def walkableInf(x: Int, y: Int) = getInf(x, y) != '#'
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// part 1
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def walk(steps: Int, starting: Set[(Int, Int)]): Set[(Int, Int)] =
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if steps == 0 then starting
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else
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// println(s"$steps => ${starting.size}")
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val ns =
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starting
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.flatMap((a, b) => Dir.all.map(_(a, b)))
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.filter(walkableInf(_, _))
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walk(steps - 1, ns)
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def part1 =
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val possible = walk(300, Set(startPoint))
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println(possible.size)
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// part 2
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object shortestPathsFrom:
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val cache = mutable.Map.empty[(Int, Int), Array[Array[Int]]]
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def apply(x: Int, y: Int) =
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cache.getOrElseUpdate(
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(x, y), {
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val mp = mutable.Map((x, y) -> 0)
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val queue = mutable.Queue((x, y))
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while !queue.isEmpty do
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val (x, y) = queue.dequeue()
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val dist = mp((x, y))
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Dir.all
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.map(_(x, y))
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.filter(walkable(_, _))
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.foreach { pt =>
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if !mp.contains(pt) then
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mp += pt -> (dist + 1)
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queue += pt
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}
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val (a, b) = mp.toMap.partition((_, dist) => dist % 2 == 0)
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Iterator(a, b)
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.map(v => v.values.toSeq.sorted.toArray)
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.toArray
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}
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)
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// input is funni
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val _ =
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assert(board.head.forall(_ == '.'))
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assert(board.last.forall(_ == '.'))
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assert(board.forall(_.head == '.'))
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assert(board.forall(_.last == '.'))
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val (x, y) = startPoint
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assert((0 until board.length).forall(board(_)(y) != '#'))
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assert((0 until board(0).length).forall(board(x)(_) != '#'))
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def pointsReachable(steps: Int): Long =
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val (sx, sy) = startPoint
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def distToRow(delta: Int) =
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if delta == 0 then 0L
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else if delta < 0 then
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// go up
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1L + sx + (delta + 1L).abs * board.size
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else 0L + (board.size - sx) + (delta - 1L) * board.size
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def distToCol(delta: Int) =
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if delta == 0 then 0L
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else if delta < 0 then
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// go left
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1L + sy + (delta + 1L).abs * board(0).size
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else 0L + (board(0).size - sy) + (delta - 1L) * board(0).size
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def startWhere(dx: Int, dy: Int) =
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val x = if dx == 0 then sx else if dx < 0 then board.size - 1 else 0
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val y = if dy == 0 then sy else if dy < 0 then board(0).size - 1 else 0
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(x, y)
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def countWhere(steps: Int, dx: Int, dy: Int) =
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val dToBoard = distToRow(dx) + distToCol(dy)
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if dToBoard > steps then 0L
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else
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val (stx, sty) = startWhere(dx, dy)
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val dists = shortestPathsFrom(stx, sty)((steps - dToBoard).toInt % 2)
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val res = dists.sortedCountUntil(steps - dToBoard.toInt).toLong
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// println(s"got $dx $dy $dToBoard $stx $sty => $res ${shortestPathsFrom(stx, sty).map(_.size).toSeq}")
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res
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(-1 to 1).zip((-1 to 1)).foreach { (dx, dy) =>
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val (stx, sty) = startWhere(dx, dy)
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// println(s"$dx $dy ${shortestPathsFrom(stx, sty).map(_.size).toSeq}")
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}
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extension (arr: Array[Int])
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def sortedCountUntil(max: Int) =
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var low = 0
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var hi = arr.size
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while low < hi do
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val mid = (low + hi) / 2
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// println(s"$mid => ${arr(mid)} ~ $max")
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if arr(mid) <= max then low = mid + 1 else hi = mid
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// println(s"found $low => ${arr(low - 1)}")
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low
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((0 to steps).toIterator
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.takeWhile(distToRow(_) <= steps)
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++ (-1 to -steps by -1).toIterator.takeWhile(distToRow(_) <= steps))
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.map(dx =>
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val dRow = distToRow(dx)
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val zero = countWhere(steps, dx, 0)
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val right =
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val (stx, sty) = startWhere(dx, 1)
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val baseDistance = dRow + distToCol(1) // dToBoard = baseDistance + (delta-1) * board.size
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if baseDistance > steps then 0L
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else
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val maxRightAllBoard =
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((steps - baseDistance - shortestPathsFrom(stx, sty).map(_.last).max) / board.size).toInt.max(0) + 1
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val odd = ((steps - baseDistance) % 2).toInt
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val a = (1L until maxRightAllBoard by 2).size * shortestPathsFrom(stx, sty)(odd).size
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val b = (2L until maxRightAllBoard by 2).size * shortestPathsFrom(stx, sty)((board.size + odd) % 2).size
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// println(
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// s"$dx => $maxRightAllBoard [${distToRow(dx) + distToCol(maxRightAllBoard)} ${shortestPathsFrom(stx, sty)
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// .map(_.last)
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// .max}] ${a + b}"
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// )
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val rest = (maxRightAllBoard to steps).toIterator
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.takeWhile(dRow + distToCol(_) <= steps)
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.map(countWhere(steps, dx, _))
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.sum
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a + b + rest
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val left =
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val (stx, sty) = startWhere(dx, -1)
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val baseDistance = dRow + distToCol(-1) // dToBoard = baseDistance + (delta-1) * board.size
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if baseDistance > steps then 0L
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else
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val maxLeftAllBoard =
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((steps - baseDistance - shortestPathsFrom(stx, sty).map(_.last).max) / board.size).toInt.max(0) + 1
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// println(s"$dx => -$maxLeftAllBoard")
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val odd = ((steps - baseDistance) % 2).toInt
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val a = (-1L until -maxLeftAllBoard by -2).size * shortestPathsFrom(stx, sty)(odd).size
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val b = (-2L until -maxLeftAllBoard by -2).size * shortestPathsFrom(stx, sty)((board.size + odd) % 2).size
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// println(s"$dx => -$maxLeftAllBoard [${distToRow(dx) + distToCol(-maxLeftAllBoard)}] ${a + b}")
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val rest = (-maxLeftAllBoard to -steps by -1).toIterator
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.takeWhile(dRow + distToCol(_) <= steps)
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.map(countWhere(steps, dx, _))
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.sum
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a + b + rest
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zero + right + left
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)
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.sum
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def part2 =
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val inputs = Seq(26501365)
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// val inputs = Seq(1, 131)
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println(inputs.map(pointsReachable))
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// val s = walk(200, Set(startPoint))
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// // divide by quadrant
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// val quads = s.toSeq
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// .groupBy((x, y) =>
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// val qx = if x < 0 then -(-x + board.size - 1) / board.size else x / board.size
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// val qy = if y < 0 then -(-y + board(0).size - 1) / board(0).size else y / board(0).size
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// (qx, qy)
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// )
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// .mapValues(_.size)
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// println((quads.toSeq, quads.values.sum))
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@main def Day21(part: Int) = part match
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case 1 => part1
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case 2 => part2
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@ -5,6 +5,8 @@ import aoc._
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object Dir:
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val dx = Array(-1, 0, 1, 0)
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val dy = Array(0, 1, 0, -1)
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val all = Array(Up, Right, Down, Left)
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import Dir._
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enum Dir:
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case Up, Right, Down, Left
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131
inputs/day21.input
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131
inputs/day21.input
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...................................................................................................................................
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.#.........#......#................#...#.......#.....##............................##..#........#.#.#.#......#......#...#........#.
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..............#..#.....#.....##....................#....................................#....##.#.......#.......#...#...........#..
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.#..#..#.#...#.....#.......#......#..#..#.............#.#..................#.......#.............#..#.......#.......#..............
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.....................#.............#............#.#................................#.......#........#...............#....#..#......
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.....#...........#.......#..................................................#..............#.##.....#..............................
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...#.........................##...#.#................#.....................................#.......#.#........#....#.......#....#..
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.#....####..........#......................#........#..................................#...........#..#..#....#......#.............
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...#.....................#........................#...............................#.......................#....#......#............
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....#......#....#......#....#....##..........................#.....#................#.....#........#.....##........................
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................#.#....#.#..#....................#..............#........................#...#.......#.#..........#.......#.#......
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.........#.#.......#............#.............#...........................................#...#.....#.......................#......
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.....#.................#.........#.#.....................#............#..........#.#.#......#...........#.#..............#...#.....
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..............................#......#.......#..#..............#....................#...#.##..##..........#...............#...##...
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.................................................................S.................................................................
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Reference in a new issue